Journal offers insights on flu pandemics back to 1889

Jul 14, 2011 (CIDRAP News) – A special supplement published by Vaccine offers some new insights about historical influenza pandemics, including how different age-groups were affected in the 1889 pandemic and how the unusual age distribution of cases in the disastrous pandemic of 1918-20 changed in the ensuing years.

The supplement, published online, includes a dozen research reports, with six of them focusing on the 1918 pandemic. The issue is supported by the Fogarty International Center of the National Institutes of Health and by the International Influenza Unit of the Office of Global Affairs at the Department of Health and Human Services.

A team of French researchers used records from five European countries and the United States to examine the age distribution of cases and deaths during the 1889 pandemic. They concluded that the clinical attack rate was "high and constant between the ages of 1 and 60 years" and lower outside that range. Further, they found that mortality rates traced a J-shaped curve, increasing with age above 20 years.

"We conclude that the age-specific attack rates of the 1889 pandemic were most similar to those of the 1968 pandemic and that influenza-related mortality rates did not follow a W-shaped curve as was observed during the 1918 pandemic," the report says, alluding to the high mortality in young and middle-aged adults in the latter pandemic.

"Adding 1889 to the short catalogue of influenza pandemics previously studied makes the 1918 pandemic even more exceptional in terms of mortality burden and age distribution of deaths.," the researchers conclude.

What happened to the age pattern of cases in the years before and after the 1918 pandemic was the focus of a study by a Danish-American team of researchers. They scrutinized Copenhagen's age-specific records of respiratory-disease deaths and influenza-like illness from 1904 to 1937.

Consistent with patterns reported elsewhere, they found that people between the ages of 15 and 64 were hit hard, while elderly people were largely spared during the pandemic years. The younger group accounted for 84% of excess mortality in 1918-19, while people over 65 had no measurable increase in deaths that season and only a moderate increase during the "recrudescent pandemic wave" of 1919-20.

Their protection was probably due to childhood exposure to a similar virus, the report says, much as was seen to be the case for older people in the 2009 H1N1 pandemic.

The researchers estimated that the age patterns of flu-related deaths returned to their pre-pandemic status, with the burden mostly on the elderly, after 1925, as younger people built up immunity.

"The first post-pandemic season associated with high excess mortality rates in individuals over 65 years was 1928-1929, with 73% of excess deaths occurring among seniors," they write. They suggest that antigenic drift (mutations) in the virus may explain the moderately increased mortality in elderly people in 1919-20 and the severe epidemic of 1928-29.

Another report in the supplement examines the burden of illness in Denmark during the 2009 H1N1 pandemic. Danish and Swedish researchers mined several surveillance data sources to estimate flu-related illness and death rates in 2009.

They estimated that 274,000 people, or 5% of the population, suffered clinical illness and that the highest attack rate was in children 5 to 14 years, at 15%. Flu-related hospital admissions in this age-group were 80% higher than expected. The number of patients admitted to intensive care units was close to 5% of the nation's capacity, and estimates of the death toll ranged from 30 to 312 (0.5 to 5.7 per 100,000 population), depending on the method used, the report says.

The researchers conclude that the pandemic brought high morbidity and "unprecedented" high rates of hospital admissions for flu-related conditions, mainly in children. "Nonetheless, the burden of illness was lower than assumed in planning scenarios, and the present pandemic compares favourably with the 20th century pandemics," they write.